Apparatus for cleaning and/or fluxing circuit card assemblies

ABSTRACT

Apparatus and a process are disclosed for cleaning and/or fluxing circuit card assemblies. The assemblies are moved through an enclosure in which they are sprayed with cleaning and/or fluxing liquid from fenestrated cylinders rotating above and below the path of the assemblies and disposed laterally thereacross. The complex spray impact pattern created by the simultaneous movements of the assemblies and the cylinders effects thorough cleaning and/or fluxing of the assemblies. Separate control of liquid pressure from the cylinders prevents unsoldered components from being dislodged from the assemblies during cleaning and or fluxing.

RELATED APPLICATION

This is a divisional of U.S. Pat. No. 4,986,462 which issued 01/22/91,Ser. No. 07/164,170 filed Mar. 2, 1988.

FIELD OF THE INVENTION

The invention herein relates to devices and methods for cleaning and/orfluxing electrical components.

BACKGROUND OF THE INVENTION

The Government has rights in this invention pursuant to Contract No.N00024-85-C-5500, awarded by the U.S. Navy.

In the electronics industry it is common to construct circuit boards byplacing the electronic components on the board in the proper positionwith respect to the printed circuitry and then to form all the solderedconnections simultaneously by use of an apparatus such as a wavesoldering device. There are however, several problems associated withthis procedure.

It is important that all connections to be soldered be properly fluxedbefore entering the wave soldering device, so that adequate solderconnections will be made. Many of the techniques now available do notallow for rapid and complete flux application.

It is also important that the printed circuitry on the board and thecontacts on the various components be clean, so that both the flux andthe solder can properly adhere. Again, the techniques currentlyavailable do not provide for rapid and complete cleaning. In fact, inmany cases, the cleaning fluids fail to reach all areas of the board orthe components or they fail to adequately remove the contaminants thatare trapped in small openings.

In addition, following the soldering step it is often necessary to cleanthe boards again to remove excess flux or solder and to be sure thatthrough holes in the board which are intended to be left open arecleaned of any solder or flux that may have closed them.

In the past there have been numerous devices to spray fluids on passingarticles. Commonly, the articles are transported through a spray chamberon a moving belt or similar device and are sprayed with fluid as theypass spray nozzles. Such devices have been used in the past to wash suchdiverse things as vegetables and paper making felts, to apply etchingsolutions to metal plates or to apply solder to electrical conductors oncircuit boards. In most cases the impact of the sprays on the articlesbeing washed, etched, soldered, etc., has not had any significant effecton the structure of the article itself or on its travel through thespray chamber. Unwanted movement of light objects such as cans on aconveyer belt under the force of the cleaning spray has been compensatedfor simply by having a second conveyer belt placed in direct proximityto the cans to limit any such motion.

Circuit card assemblies pose a much more difficult problem, however. Thevarious components are placed on the board but are not secured firmly intheir positions until the soldering step has been completed.Consequently, the cleaning and fluxing steps before the soldering mustbe done with considerable care so that the components are not dislodgedfrom the proper positions. Many of the cleaning devices heretofore knownhave been unable to properly clean or flux such circuit card assemblieswithout causing significant amounts of dislodging and dislocation of thecomponents.

It would therefore be of considerable interest to have a device whichcould readily clean and flux circuit card assemblies without risk ofdislocating or dislodging any of the components. It would also be ofinterest to have such a device which could effectively clean and fluxthe circuit card assemblies by insuring that the cleaning or fluxingfluids would penetrate into all parts of the assembly, thoroughly removecontaminants and apply flux to the appropriate contacts. It would alsobe advantageous for such a device to be capable of being used followingthe soldering of the circuit card assemblies to remove excess flux andsolder and to insure that all through passages were properly opened.

BRIEF SUMMARY OF THE INVENTION

The invention herein comprises apparatus to clean or flux circuit cardassemblies with a liquid. In its broadest form the apparatus comprises asubstantially liquid-tight enclosure with an entrance opening and anexit opening communicating with the interior of the enclosure; means tomove the circuit card assemblies along a substantially horizontal paththrough the interior of the enclosure with entry to and exit from theinterior via the respective openings; rotating liquid dispensing meansdisposed above and below the horizontal path and extending laterallythereacross, the dispensing means having separate portions above andbelow the path adapted to dispense the liquid in a plurality of spraysdirected toward said path from above and below the path; means to rotatethe dispensing means; conduit means to convey the liquid separately tothe portions of the dispensing means under sufficient pressure to causethe sprays to be dispensed and a plurality of pressure regulation meansdisposed in the conduit means to separate control of the pressure ofsaid liquid in each of said portions and adapted to permit the liquid inthe sprays above said path to be controlled to a different pressure fromthe liquid in the sprays below the path.

In preferred embodiments the dispensing means comprises two rotatinghollow cylinders with perforated surfaces, one disposed above the pathand the other disposed below the path. In other embodiments theapparatus also contains means for collecting the liquid that isdispensed in the sprays, filtering it and recycling it to the dispensingmeans so that it can be reused for additional washing or fluxing. Instill other embodiments the invention comprises means for heating theliquid prior to dispensing it as a spray.

In another aspect, the invention is a process for cleaning or fluxingcircuit card assemblies with a liquid; moving the assemblies through asubstantially liquid-tight enclosure on a generally horizontal pathextending from one end of the enclosure to the other end; spraying theassemblies with the liquid simultaneously from above and below the pathwhile the assemblies move along the path, the sprays being dispensedfrom rotating dispensing means whose linear surface speed parallel tothe path is greater than the speed of travel of the assemblies along thepath such that the instantaneous angle at which spray encounters theassemblies varies as a function of both the linear travel of theassemblies along the path and the rotation of the spray dispensingmeans; and draining a substantial portion of the liquid from theassemblies. The cleaning/fluxing process is preferably followed promptlyby a soldering step to solder the electrical contacts on the assemblies.If desired, a cleaning process substantially the same as thecleaning/fluxing process but using only cleaning liquid, may follow thesoldering step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the device of this invention.

FIG. 2 is a sectional side elevation view taken on line 2--2 of FIG. 1.

FIG. 3 is a schematic diagram of the recycle and pressure regulationsystem of this invention.

FIG. 4 is a sectional and elevation view taken on line 4--4 of FIG. 2.

FIG. 5 is a detail view of a portion of the surface of one cylindertaken on line 5--5 of FIG. 4.

FIG. 6 is a sectional view of the surface portion taken on line 6--6, ofFIG. 4.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The nature of the device is best understood by reference to thedrawings. The overall configuration is illustrated in FIGS. 1 and 2. Ahousing 10 forms an interior enclosure 12 in which is defined asubstantially horizontal path 18 between them. In preferred practicethis device is positioned immediately adjacent to and ahead of a wavesoldering system such that cleaned and fluxed circuit card assembliesexiting through exit port 16 pass directly into the wave solderingapparatus, thus minimizing any effect of flux drainage and anyopportunity for the circuit card assembly to become re-contaminated.

Passing through the enclosure 12 and entry and exit ports 14 and 16 isconveyer line 20. The particular type of conveyer line 20 which is usedwill depend on the particular circuit card assemblies to be treated, butin each case the conveyer line 20 will have sufficient openings orperforations such that spray liquids can adequately reach the undersideof each of the circuit card assemblies. Normally the conveyer line 20will have a width just slightly less than the interior width of theenclosure 12, as illustrated in FIG. 1, in order to utilize theapparatus of this invention in an optimum manner.

The conveyer line 20 may be driven by any convenient means, such as thecontinuous drive chain 22 which is journaled around pulleys 24, at leastone of which will be powered by a suitable motor 25. The drive chain 22or similar driving device will be controlled such that its speed willmatch the speed of the downstream wave soldering device to insure thatthe circuit card assemblies are moved smoothly and continuously throughboth devices.

Mounted in the enclosure 12 are two rotating drums 30 and 32 whichextend laterally completely across the enclosure 12 and are disposed onopposite sides of the conveyer line 20. Each of the drums 30 and 32 is ahollow cylinder closed at the ends (except as noted below) and having afenestrated cylindrical surface 34. The number and size of the openings36 on surface 34 will be dependent on several factors, including theviscosity of the fluid to be sprayed through the openings, the speed atwhich the drums 30 and 32 rotate, the pressure under which the fluid isdispensed as spray, and the type of circuit card assemblies which are tobe cleaned and/or fluxed. In a typical embodiment the drums 30 and 32will be approximately 6" in diameter with lengths of approximately 16"yielding drum surfaces of approximately 600 sq. inches. Spray openings36 will be placed at approximately 1" intervals both longitudinally andradially around the cylindrical surface 34, and will have diameters ofapproximately 1/4". On the outer side of the surface 34 the holes 36will preferably flare outward in a lens shaped pattern 37 as shown inFIGS. 5 and 6 in order to insure thorough dispersion of the spray andcomplete coverage of the passing circuit card assemblies. This flaredhole shape can be advantageously used to dispose a mixed liquid offluorinated hydrocarbon cleaner and soldering flux.

Each of the cylinders 30 and 32 has an axle 40 protruding from one endthereof which is journaled in bearing 42 mounted on the side of housing10. Also mounted on axle 40 is pulley 44 around which runs drive belt 46to drive motor 48. At the other end of drum 30 or 32 is hollowcylindrical conduit 50 or 60 respectively which is journaled in bearing52 mounted on housing 10. The two separate conduits 50 and 60communicate respectively with the interiors of drums 30 and 32. Thepurpose of having two separate conduits 50 and 60 is to permit separatecontrol of the sprays in each drum 30 or 32, as will be discussed below.The conduits 50 and 60 meet at tee 62 to which is joined main fluid line64. Mounted within each of the lines 50 and 60 are pressure controllers66 and 68 respectively which can be individually adjusted to controlseparately the fluid sprays being emitted from holes 36.

The bottom 70 of housing 10 is preferably shaped to create a sump 72 atthe bottom of the enclosure 12 so that excess spray liquid 75 which isnot retained on the circuit card assemblies can be collected and cleanedfor recycle and reuse. Mounted at or near the bottom of sump 72 is drainpipe 74 which preferably is capped by a screen 76 to trap largecontaminant particles. The removed fluid passes through drain pipe 74which preferably is a replaceable cartridge filter. The filter mediumwill be selected to have a porosity adequate to the removed contaminantparticles but yet allow proper flow of the cleaning and/or fluxingliquids to the recycle system. The recycle system consists of conduit 80which passes from the filter to a high pressure fluid pump 82 which isdriven by pump motor 84. The pump discharges at high pressure into mainfluid feed line 64. Line 64 also contains main pressure regulator 65which is used to control the overall fluid pressure prior to passing thefluid to conduits 50 and 60. Either or both of the cleaning and/orfluxing liquids can be replenished in the system by feeding inadditional fluid through feed line 90 which is controlled by valve 92and which leads from suitable supply reservoirs (not shown). Theenclosure 12 may also be partially or completely drained if desiredthrough drain line 74 and second drain line 96 which is controlled byvalve 98 and which leads to collection facilities (not shown).

Also preferably mounted in sump 72 is a heater element 100 which can beused to maintain a desired temperature of the collected liquids so thatcoagulation or separation of the liquids can be prevented. This is alsoused to maintain the optimum liquid temperature for cleaning or fluxingthe circuit card assemblies.

The operation of the device is highly efficient and reflective of theunique structure of the apparatus. As the circuit card assemblies 102are moved into the enclosure 12 on conveyer line 20 they immediatelybegin to be drenched with spray 104 from the rotating drums 30 and 32.As they move closer to the drums 30 and 32 and ultimately pass betweenthem, the amount of spray 104 received increases as does the impactforce of the spray 134 against the circuit card assemblies 102. As theassemblies 102 continue to move past the drums 30 and 32 toward exitport 16 the amount and force of the sprays 104 impacting on the circuitcard assemblies 102 diminishes. The rotation of the drums 30 and 32combined with the movement of the assemblies 102 on conveyer line 20produce the highly sufficient washing and fluxing action of the sprays.It will be evident that as both the cylinders 30 and 32 rotate and theassemblies 102 move horizontally along line 20, the angle at which thesprays 104 hit the assemblies 102 constantly changes in a pattern morecomplex than a simple pattern obtained by passing an assembly under astationary spray. This highly complex pattern insures that the spray isnot shielded from reaching any part of the assembly by the components onthe assembly, since the rotation of the cylinders 30 and 32 serves tofollow the assemblies 102 as they pass. Since the cylinders 30 and 32normally rotate faster than the conveyer line 20 moves, however, theeffect is to have the sprays 104 wash over the assemblies 102 in athorough sweeping motion a number of times as the assemblies 102 movethrough the enclosure 12. This repeated sweeping motion is not obtainedfrom stationary sprays or from rotating sprays with stationary articles.This system, therefore, by providing the dual motions of the cylindersand the assemblies, produces a unique washing or fluxing system which issignificantly more thorough than has been obtainable with prior artdevices. In a typical example, using the dimensions exemplified above,the cylinders rotate at approximately 50 RPM giving a linear surfacespeed relative to the path of the assemblies of approximately 160 feetper minute. The assemblies, on the other hand, move at approximately 4feet per minute through the chamber, thus giving a relative washing rateof 40 "sweeps of" the wash or flux liquid across the boards as they passthrough the enclosure. This is to be contrasted with the single "sweep"which is obtained when a moving assembly passes a stationary spray or amoving spray passes a stationary article.

Not only are the assemblies 102 swept by the wash and/or flux liquidsfar more times during a single pass through the unit than is possiblewith the prior art devices, but in addition the multiple sweeps resultin much more effective cleaning. This is because each sweep tends todissolve and dislodge a certain amount of contaminant and leaves thatamount of contaminant suspended in the liquid adhering to the assembly.The next sweep tends to wash away that liquid with the contaminantcontained therein. It also serves to dissolve and dislodge the nextlayer of contaminant to be followed by another sweep which washes awaythe materials dislodged by the second sweep, and so on. A device whichprovides only a single sweep obviously cannot be as effective in fullyremoving the contaminants or in dispersing the flux to all surfaces tobe soldered. In addition, by combining flux and wash in a single liquidmixture, the early sweeps during passage will serve to be more effectivein cleaning while the subsequent sweeps will be more effective inapplying flux to the previously cleaned surfaces.

It will be understood that the mention of "sweeps" in this specificationare not meant to imply that there must be individual waves of liquidacross the assemblies interspersed with periods in which no liquid isbeing sprayed on the assemblies. That is of course a possible method ofoperation of the device and could easily be accomplished by havingthesurfaces the surfaces the drums not fully fenestrated so that alongitudinal band of holes is followed by a band in which no holesoccur. It is preferred, however, to have the surface uniformlyfenestrated so that spray is essentially continuous against theassemblies as they pass through the enclosure. In either context, theterm "sweep" as used herein is defined as one full revolution of thedrum. Thus one "sweep" will bring all openings on the drum surface intodirect proximity to the passing assemblies, regardless of whether theopenings are evenly spaced across the surface of the drum or clusteredin groups or bands. Thus in the example presented above, the cylinderswill make forty complete revolutions ("sweeps") relative to theassemblies each minute.

The second critical aspect of the present invention, in addition to thedual rotation described above, is the variable pressure from the spraysbelow or above the moving assemblies 102. As noted earlier, the circuitcard assemblies 102 reach the enclosure with the various electroniccomponents merely fitted into their mounting holes but not securedtherein. While they are sufficiently tightly fitted to maintain theirpositions during the motion along the conveyer line 20, they are stillsusceptible to being dislodged from the mounting holes if the force ofthe sprays is greater than the frictional force holding the componentsin position. The sprays, on the other hand, must be of sufficient forceto wash the liquids into all parts of the assemblies and dislodge anycontaminants, as well as spreading flux to all required surfaces. Thisdilemma is resolved in the present apparatus by the separate pressurecontrols on the liquids passing respectively to drums 30 and 32. A muchstronger force can thus be applied against the upper side of theassemblies 102 by the spray from drum 30. The components can withstandgreater impact from this direction since it merely pushes them morefirmly into their mounting holes. The spray from the underside, however,will be at a reduced pressure to avoid pushing the components up and outof their mounting holes while still providing adequate washing andfluxing action for the underside of the board.

The particular pressures used will depend on the fluids involved andtheir viscosity as well as the type of boards being cleaned. Typicallythe overall system pressure created by the high pressure pump isapproximately 400 psi. The main pressure controller 65 maintains thepressure at about 200-400 psi in line 50. The pressure is furtherreduced to about 50-100 psi in the upper cylinder 30 by pressurecontroller 66 and about 50-100 psi in the lower cylinder 32 by pressurecontroller 68. The specific pressures selected will depend on boardconfiguration, conveyer line speed and type and location of componentsinstalled on the board. Those skilled in the art will be easily able toselect appropriate pressures for specific operations.

It would also be possible if desired to have additional cylindersmounted in the enclosure, either in pairs as 30 and 32 or with morecylinders on one side of the path 18 than on the other. However, in viewof the thorough washing and fluxing which can be obtained with a singlepair of cylinders, additional cylinders are not usually necessary.

If it is desired to have a series of washings or fluxings so that thefluids do not mix, then it is necessary to have a plurality of thedevices of this invention arranged seriatim along the path 18, each withits own conveyer line 20, to allow each separate washing or fluxing tooccur and be completed before the next one takes place.

It is also possible to use the device of this invention immediatelyfollowing the wave solder unit for washing only, since fluxing is nolonger needed. In this "downstream" device the cylinders would dispenseonly washing liquid which would clean away the excess flux and solder aswell as insuring that all through holes were satisfactorily opened ifthey should have inadvertently closed during the soldering procedure.

The device of this invention may be made of any convenient materialwhich will withstand the effects of the spray compositions and the fluidpressures involved. Typically a stainless steel with appropriate rubberor other synthetic gaskets and seals will be quite adequate, although itis possible that some of the more resistant plastics may be used forsuch things as the housing. Rubber will also be used in the splashcurtains 110 which are at each of the exit and entry ports 14 and 16.The combination of the housing and the splash curtains serve to make theentire enclosure substantially liquid-tight to minimize the amount ofmist or spray which escapes as the assemblies pass into or out of theenclosure through the ports 14 and 16. The splash curtains 110 need notbe located in line with the housing wall as shown in FIG. 2, if there isan extension such as 112 aligned with either the exit port 14 or entryport 16. In fact if such extensions are used it may be possible todispense with the splash curtains altogether, depending on thevolatility of the liquids and the amount of spray which enters theextension.

FIG. 2 also shows optional splash guards 106 surrounding the portions ofthe drums where the sprays 104 are not directed at the moving assemblies102. These splash guards 106 tend to deflect spray back in the directionof the assemblies and also to minimize the amount of spray contactingthe interior walls of the enclosure. This in turn minimizes the amountof corrosion of the housing wall or the frequency with which the housingwall needs to be cleaned or otherwise maintained.

Also preferably included in the housing is fluid level float sensor 114.This is placed at the point of the desired upper surface of the liquid75 in the sump 72 and will indicate to the operator when the fluid isabove or below that level, thus alerting the operator to the end todrain or replenish the fluid supply. Normally it will be a matter ofreplenishing the supply since some of the fluid will evaporate, be lostby spray exiting the enclosure or be carried off as flux and wash liquidadhering to the circuit card assemblies. It is desirable that the sensorbe one that detects a range of fluid levels so that both the maximum andminimum quantities of liquid desirably in the sump 72 can be controlled.

It will be evident that there are numerous embodiments of this inventionwhich while not expressly illustrated above are clearly within the scopeand spirit of the invention. The above disclosure is therefore intendedto be exemplary only and the proper scope of the invention is to bedefined solely by the appended claims.

I claim:
 1. Apparatus to clean or flux circuit card assemblies with aliquid, which comprises:a substantially liquid-tight enclosure with anentrance opening and an exit opening communicating with the interiorthereof; means to move said circuit card assemblies along asubstantially horizontal path through said interior with entry to andexit from said interior via said respective openings; rotating liquiddispensing means disposed above and below said path and extendinglaterally thereacross, said dispensing means having separate portionsabove and below said path adapted to dispense said liquid in a pluralityof sprays directed toward said path from above and below said path;means to rotate said dispensing means; conduit means to convey saidliquid separately to said portions of said dispensing means undersufficient pressure to cause said sprays to be dispensed; and aplurality of pressure regulation means disposed in said conduit means toseparate control of the pressure of said liquid in each of said portionsand adapted to permit the liquid in said sprays above said path to becontrolled to a different pressure from the liquid in said sprays belowsaid path.
 2. Apparatus as in claim 1 wherein said dispensing meanscomprises two rotating hollow cylinders with perforated surfaces, onedisposed above said path and the other disposed below said path. 3.Apparatus as in claim 1 further comprising means to collect said liquiddispensed in said sprays and to filter and recycle it to said dispensingmeans.
 4. Apparatus as in claim 1 further comprising means to heat saidliquid.
 5. Apparatus as in claim 1 wherein said conduit is adapted toconvey a liquid comprising a fluorinated hydrocarbon.
 6. Apparatus as inclaim 5 wherein said conduit is adapted to convey a liquid comprising amixture of a fluorinated hydrocarbon and a soldering flux.
 7. Apparatusas in claim 1 further comprising a wave soldering device following saidapparatus and means to convey said circuit card assemblies to saiddevice immediately after passage through said apparatus.
 8. Apparatus asin claim 1 further comprising a wave soldering device preceding saidapparatus and means to convey said circuit card assemblies from saiddevice to said apparatus immediately after passage through said device.9. Apparatus as in claim 1 wherein said means to rotate comprises amotor.